1. Field of the Invention
The present invention relates to a semiconductor device and, in particular, to a semiconductor device capable of reducing a short-channel effect.
2. Description of the Related Art
FIG. 1 illustrates the structure of a conventional semiconductor device or transistor. As illustrated in the drawing, active regions 101 and inactive regions 102 are formed on a semiconductor substrate 100. A word line 103 having a certain width is formed on an upper surface of the semiconductor device 100. The word line 103 includes a pattern of lines carrying a gate oxide film (not shown). The word line 103 crosses the active region 101 in a vertical direction. The word line 103 divides the active region 101 to form a source 104 and a drain 105. In addition, the intersection of the word line 103 with the active region 101 forms a gate electrode 103a in an upper part of the active region 101. A channel is formed when a certain voltage is applied to the gate electrode 103a. 
As shown in FIG. 1, Lc is a gate length of the center of the channel, and Le is a gate length of the sides of the channel. In the conventional transistor of FIG. 1, the gate length (Lc) of the center of the channel is the same as the gate length (Le) of the sides of the channel. The conventional transistor having the channel with this type of shape includes a number of disadvantages after implanting impurity ions and annealing the transistor.
FIG. 2 illustrates the conventional transistor after the formation of a lightly doped drain (LDD). Impurity ions are implanted in a portion of the gate electrode 103a. Annealing the transistor diffuses the implanted impurity ions down to the gate electrode 103a, as represented by portions 201. As seen in the drawing, diffusion of the impurity ions occurs more extensively at the corners of the channel than at the center of the channel. A transistor formed by this process has an effective length of the channel of the corners (Leffe) that is relatively shorter than an effective length of the channel of the center (Leffc). As a result, a leakage current is increased and a threshold voltage is lowered at the corners of the channel. For these reasons, it is desirable to form a transistor having an effective length of a channel of the corners that is about equal to an effective length of a channel of the center.